In recent years, fiber optic technology has seen increasing uses in a variety of widely diverse fields. The use of this technology in many fields requires that the coupling efficiency of the fiber, i.e. the ability to gather light directed at the fiber end, be as high as possible. With simple butt coupling techniques, less than one percent of the energy emitted from a light emitting diode can be launched into a single mode fiber. With a laser diode source, the figure is approximately ten percent. The reason for this poor performance is the modal mismatch at the fiber end. With a lens attached to the fiber end, however, these coupling efficiencies can be improved dramatically. These improvements lead to commensurate improvements in transmission distance and signal-to-noise ratio.
Fiber optic technology also finds application in various medical fields, such as endoscopy and laser surgery. These applications, too, rely on the use of lenses on fiber ends, here principally for purposes of minimizing optical distortion and focusing laser energy.
Several techniques for forming lenses at optical ends have previously been reported. One class of techniques involves the attachment of a discrete lens to the fiber. An example of this technique is shown in U.S. Pat. No. 4,269,648 to Dackss et al., wherein a microsphere bead is glued to a cleaved fiber end with a transparent glue. The Dackss et al. technique is claimed to center the lens on the fiber core to within five microns. Another example of a discrete attachment technique is shown in U.S. Pat. No. 4,380,365 to Gross, wherein a microsphere bead is positioned on the end of a cleaved fiber and is melted in place. Still another example of a discrete attachment technique is shown in U.S. Pat. Nos 4,118,270 to Pan et al. and 4,067,937 to Unno et al., wherein a cleaved fiber end is dipped in molten glass or epoxy to form a hemispherical lens thereon.
A second class of techniques for providing a lens at an optical fiber end involves direct fabrication of a lens onto the fiber. This approach is generally preferable to the discrete lens attachment approach becauseit has the advantage of relative mechanical simplicity and freedom from complicated alignment procedures.
One such direct fabrication technique involves cleaving the fiber to a square edge and then etching the end of the fiber in a hydrofluoric acid solution to form a rounded lens thereon. The dimensions of the lens are a function of the concentration of the acid, the acid bath temperature and the duration of the chemical etch. An example of this technique is shown in U.S. Pat. No. 4,118,270 to Pan et al. Additional examples of the technique are discussed in the following articles: "Chemically Etched Conical Microlenses for Coupling Single-Mode Lasers Into Single-Mode Fibers," by Eisenstein et al., Applied Optics Vol. 21, pp. 3470-3474 (Oct. 1, 1982); "Improved Coupling Between Laser Diode and Single-Mode Fibre Tipped With a Chemically Etched Self-Centered Defracting Element," by Kayoun et al., Electronic Letters Vol. 17, pp. 400-402 (Aug. 9, 1985); and "Microlens Formation on VAD Single-Mode Fibre Ends," by Kawachi et al., Electronic Letters, Vol. 18, pp. 71-72 (Jan. 21, 1982).
A second technique for fabricating a microlens directly on the end of an optical fiber is to heat the fiber in an electric arc or flame and pull its ends so as to form a narrow waist. The fiber can then be cleaved at this waist or it can be heated further until the waist separates. In both cases, a long, substantially conically tapered lens results. Examples of this technique are shown in U.S. Pat. No. 4,589,897 to Mathyssek et al. and in British Patent No. 2,110,835 to Bricheno. The technique is additionally discussed in an article entitled "Efficient Coupling from Semiconductor Lasers Into Single-Mode Fibers with Tapered Hemispherical Ends," by Kuwahara et al., Applied Optics Vol 19, pp. 2578-2783 (Aug 1, 1980).
Still another technique for forming a microlens directly on a fiber end, or for rounding a microlens formed by another technique, is to heat the end of the fiber to its melting point. As the glass melts, surface tension acts to minimize the end's surface area, thereby producing a rounded surface.
A variety of common problems plague all of the above techniques, making them poorly suited for practical application. For example, common to all of these techniques is the problem of repeatability. In many applications, it is important to form identical lenses on a plurality of fibers. Such repeatability has heretofore been unattainable.
Another related common problem is that of achieving the precise lens configuration desired. Most lensing techniques can be modelled mathematically to determine the parameters necessary to achieve a lens of the desired shape. However, these parameters cannot be implemented with the degree of precision necessary to accurately obtain the desired lens. For example, the drawn taper and flame rounding techniques operate as functions of heat distribution, glass composition and, in the case of the drawn taper, the pulling force. However, these factors, particularly the heat distribution factor, cannot be sufficiently controlled to produce a lens having the precise configuration desired. Similarly, the dimensions of lenses formed by chemical etching can only be grossly controlled by the acid concentration, acid temperature and bath duration factors. In actual practice, the resulting lenses only roughly approximate the desired configuration.
This problem of deviation of lens geometry from the desired shape, or lens aberration, has been recognized as the dominant component of coupling loss in a laser-fiber junction by Sumida et al. in an article entitled "Lens Coupling of Laser Diodes to Single-Mode Fibers," Journal of Light Wave Technology Vol. 2, pp. 30514 -311 (June, 1984). Consequently, reduction of lens aberrations is a primary concern in the development of a highly efficient coupling.
Still another common problem is that of dealing with fibers having unusual geometries. Although most fibers have a circular core positioned in the center of the cladding, other fiber geometries are also used. For example, some fibers have cores that are not circular in cross section or that are not centered in the cladding. Prior art lensing techniques are grossly unsuited for use with fibers having such properties.
Yet another common problem is that of dealing with polarization preserving fibers. Such fibers typically include regions adjacent the core that are highly doped with alumina to induce a stress in the fiber that aids in birefringence. These highly doped regions do not etch at the same rate as glass in chemical etching techniques, nor do they melt at the same temperature as glass in drawn taper techniques. This doping inhomogeneity thus interferes with the formation of lenses on these fibers by known techniques.
In addition to the common problems noted above, each of the prior art lensing techniques also has its own peculiar drawbacks. The chemical etching technique, for example, requires the use of highly reactive solutions, such as hydrofluoric acid, to etch away the glass. The use of such solutions renders the technique unsuitable for all but the most carefully controlled laboratory conditions. The technique also suffers in that only a narrow range of lens geometries can be formed thereby.
The problems peculiar to the drawn taper technique are numerous. One is the geometry of the resulting lens. In the tapered lens region, the core and cladding thicknesses both decrease. Because of this tapering, the core diameter and the light collection angle are both reduced, so the chance of light coupling into the cladding layer is increased.
Another drawback peculiar to the drawn taper technique is that different softening points in the core and cladding material may result in unevenness on the fiber surface, leading to additional scattering losses from the light source.
Still another problem of the drawn taper technique is that of lens centering. In an exemplary single mode fiber, the core may be only eight microns in diameter. Consequently, a lens must be centered on the fiber to within a small fraction of this figure if it is to operate efficiently. Such positioning precisions cannot reliably be obtained by this technique.
Yet another drawback of the drawn taper technique is that the composition of the lens formed thereby is unpredictable. The parting of the fiber at its narrow waist leaves the core surrounded by cladding at the lens end. When flamed to round the lens, this cladding material mixes with the core material so that the lens formed therefrom is of unpredictable composition.
A final drawback of the drawn taper technique is that the range of geometries attainable is limited to long tapers. The technique is thus poorly suited for applications in which a longer focal length, and thus a shorter taper is required.
In addition to the lensing techniques discussed above, all of which have found at least some measure of practical application, a multitude of other lensing techniques have been proposed but have proved entirely unworkable in practice. Exemplary of this class of techniques is mechanical polishing of lens surfaces into pyramid form, as referenced in the article by Sakaguchi entitled "Power Coupling From Laser Diodes Into Single-Mode Fibres With Quadrangular Pyramid-Shaped Hemiellipsoidal Ends," Electronic Letters, Vol. 17, pp. 425-6 (June 11, 1981). The fatal problem in this technique was apparently the requirement that the polishing apparatus be positioned and aligned relative to the fiber to within micron-order tolerances, a requirement that is unattainable in practice. The prior art is replete with other such instances of lensing techniques that are entirely unsuited for practical application.
From the foregoing it will be recognized that a long felt need still exists for a method and apparatus for providing microlenses on optical fibers which overcomes the above-noted drawbacks in the prior art. The present invention fulfills this need.
According to the present invention, an end of an optical fiber is urged against a moving abrasive lap at a desired angle and is simultaneously turned, either continuously or in discrete steps. The turning causes the fiber end to contact the abrasive at points all around its periphery, thereby removing material equally from all sides of the fiber and producing a precise, well-centered lens form. By varying the duration of contact on various surfaces, lenses of various configurations, such as conical, elliptical, pyramidal, truncated pyramidal, bevelled cleave, etc., can be obtained. Novel techniques are employed inter alia, to regulate the grinding pressure and to center the lens on the fiber.
The features and advantages of the present invention will be more readily apparent from the following detailed description of a preferred embodiment thereof, which proceeds with reference to the accompanying drawings.